1. Technical Field
An antenna device for use in a compact mobile telephone or the like and capable of multiple-resonance wideband transmission and reception, and to a wireless communication apparatus.
2. Background Art
In the related art, antenna devices of this type include the antenna devices shown in FIGS. 19 to 21.
FIG. 19 is a plan view showing a multiple-resonance antenna device of the related art, FIG. 20 is a plan view of a wideband antenna device of the related art, and FIG. 21 is a plan view showing a multiple-resonance wideband antenna device of the related art.
First, the antenna device 100 shown in FIG. 19 is an inverted-F-shaped antenna device as disclosed in Patent Document 1. The antenna device 100 has a structure in which a plurality of additional radiation electrodes 111 to 113 which are grounded are connected to a radiation electrode 101 through switches 121 to 123.
The antenna device 100 is therefore an antenna device in which a plurality of resonant frequencies can be selected by switching the switches 121 to 123 to achieve multiple resonances.
Next, the antenna device 200 shown in FIG. 20 is an inverted-F-shaped antenna device as disclosed in either Patent Document 2 or 3. The antenna device 200 has a structure in which an additional radiation electrode 210 is branched from a radiation electrode 201 and in which a variable capacitance element 211 is connected to a distal end of the additional radiation electrode 210 and is grounded.
The antenna device 200 is therefore an antenna device in which a resonant frequency can be shifted by changing an impedance of the variable capacitance element 211 to achieve a wide resonant frequency band.
Finally, the antenna device 300 shown in FIG. 21 is an antenna device as disclosed in Patent Document 4. The antenna device 300 has a structure in which a plurality of additional radiation electrodes 311 and 312 which are grounded are connected through switches 321 and 322 to a radiation electrode 301 whose distal end is grounded and in which variable capacitance elements 331 (and 332) are provided in the additional radiation electrode 311 (and 312).
The antenna device 300 is therefore an antenna device in which a plurality of resonant frequencies can be selected by switching the switches 321 and 322 to achieve multiple resonances and in which resonant frequencies can be shifted by changing impedances of the variable capacitance elements 331 (and 332) to increase the bandwidth of the resonant frequencies.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-261533
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2005-210568
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2002-335117
Patent Document 4: International Publication No. WO 2004/047223
However, the antenna devices of the related art described above have the following problems.
The antenna device 100 shown in FIG. 19 suffers from significant degradation of antenna gain.
In general, in compact antenna devices, the use of a lower resonant frequency decreases antenna gain, resulting in degradation of antenna efficiency. The antenna device 100 shown in FIG. 19 is configured to obtain the lowest resonant frequency by turning on the switch 123. In such a situation, loss due to the switching operation reduces antenna gain, resulting in further degradation of antenna efficiency.
In the antenna device 100, further, a current flows to the additional radiation electrode corresponding to the switch that is the closest to a feed unit the among switches that are in the on state. For example, even when all the additional radiation electrodes 111 to 113 are turned on, a current flows only in the switch 121, which is the closest to a feed unit 400, and no current flows in the switch 122 or 123. Further, only a number of resonant frequencies corresponding to the number of switches 121 to 123 are generated, so that the number of resonant frequencies is small.
The antenna device 200 shown in FIG. 20 also suffers from degradation of antenna efficiency.
In the antenna device 200, since only the variable capacitance element 211 is grounded, the minimum voltage is at the variable capacitance element 211 and a maximum current flows in the variable capacitance element 211. Power consumption at the variable capacitance element 211 becomes large, resulting in great degradation of antenna efficiency.
In the antenna device 300 shown in FIG. 21, it is difficult to reduce the antenna size.
In the antenna device 300, a maximum voltage is generated on the radiation electrode 301, which is parallel to a ground region 402, but is not generated near the feed unit 400. A minimum voltage is generated at the distal end of the radiation electrode 301. Thus, the antenna device 300 operates only at an antenna length equal to a half wavelength but does not operate at an antenna length equal to a quarter wavelength. The radiation electrode 301 is therefore long, and a reduction in antenna size is not achieved.
In the antenna device 300, further, it is difficult to match impedance between the feed unit side and the antenna side at all frequencies.
The impedance of the antenna device 300 is determined by taking stray capacitance generated between the radiation electrode 301 and the ground region 402 into account. The switching operation of the switches 321 and 322 causes a change in a maximum electric field position each time the switching operation is performed. Thus, the capacitance component of the impedance greatly varies depending on antenna installation conditions. As a consequence, depending on the switching state of the switches 321 and 322, matching between the feed unit 400 side and the antenna is or is not achieved, and accurate matching at all resonant frequencies is not achieved.